1. Field of the Invention
The present invention relates to a valve structure installed in an engine exhaust system.
2. Description of the Related Art
Conventionally, as a valve of this kind, a structure is known in which a valve is installed in a pipe passage within a muffler of the vehicle.
This structure improves the silencer effect by opening or closing an exhaust passage corresponding to a variation in pressure of an exhaust gas flowing in from the engine side, and prevents a decrease in the engine output. In such an exhaust pressure sensitive valve, a spring is annexed for biasing a valve disk in a closing direction, and the valve is opened when an exhaust pressure enough to overcome a biasing force of the spring is exerted.
By the way, in an environment where high pressure exhaust gases flow, a hinge portion for carrying the valve disk can not be fitted minutely in view of an expansion or shrinkage due to thermal expansion, the precision of the rotation axis being relatively rough. Hence, if a contact surface between the valve disk and a valve seat is inclined conically, for example, the center between them is dislocated even in a state where the valve is closed, developing a gap in practice.
Accordingly, the contact surface between the valve disk and the valve seat is made planar, as shown in FIG. 10.
In the valve structure as shown in the figure, a support member 10 extending from a muffler pipe 1 has a flange 12 and forms the valve seat. The valve disk 40 carried around a hinge axis 15 by a support bracket 14 of the support member 10 has an entire smooth face that is opposed to the flange 12 of the support member, with a flange 41 around its peripheral edge. A spring 16 is wound around the hinge axis 15, and biases the valve disk 40 toward the flange 12 that is the valve seat.
The valve disk 40 is opened when the exhaust pressure is increased, or closed when the exhaust pressure is decreased. In the case where the valve disk is made from a thin sheet by press molding, the resonance frequency is higher due to a combined effect between the valve disk and the spring 16 annexed, so that the valve disk 40 is caused to vibrate at an engine speed of 2000 to 3000 rpm in a normal range of the engine, leading to a problem that beats are produced and the quality sense of the vehicle is deteriorated.
To suppress the vibration of the valve disk, it is conceived to increase a biasing force of the spring 16 for biasing the valve disk 40 in the closing direction. However, if the spring force is increased, the valve may not be opened till the exhaust pressure becomes a considerably high value, so that the proper object can not be attained.
On the other hand, if the spring force is reduced to make the resonance frequency lower, the opening valve pressure for opening the valve is decreased, so that the valve is frequently opened or closed due to slight variations in exhaust pressure because the valve disk 40 itself is light in weight.
Accordingly, the invention has been achieved in the light of the above-mentioned problems associated with the conventional art, and it is an object of the invention to provide a valve structure for an engine exhaust system in which vibration of a valve disk due to resonance is suppressed in the case where the valve disk is made from a thin sheet and light in weight.
According to the invention, there is provided a valve structure for an engine exhaust system in which a valve installed in an exhaust passage is opened or closed in accordance with an exhaust pressure, with a valve disk biased in a closing direction by a spring being supported around a rotation axis, wherein the valve disk is formed with a bead portion in a region excluding a contact surface of a valve wall with a valve seat, and a weight is fixed to the bead portion.
Since the weight is fixed to the valve wall, the resonance frequency of the valve disk can be changed while the predetermined characteristics of the spring are retained.
The valve wall of the valve disk has an outer peripheral portion shaped like a ring as the contact surface, the bead portion has an inner diameter portion projecting out to the back side from the outer peripheral portion, and the weight is fixed to the bead portion on a back face of the valve wall. Since the weight is fixed to the back face of the valve wall, there is no risk of interference with the valve seat.
The spring is wound around the rotation axis, with one end engaging the back face of the valve wall, and the weight is fixed except for an engagement point at one end of the spring. Since the engagement point of the spring is avoided, there is no risk that one end of the spring is engaged on the weight accidentally.
According to the invention, there is provided a valve structure for an engine exhaust system in which a valve installed in an exhaust passage is opened or closed in accordance with an exhaust pressure, a valve disk biased in a closing direction by a spring is supported around a rotation axis, wherein the valve disk has a weight of circular arc fixed on a back face along an outer peripheral edge including an arcuate portion most left away from the rotation axis of a valve wall.
Since the weight is fixed to the valve wall, the resonance frequency of the valve disk can be changed while the spring is kept at the predetermined characteristics. Particularly, since the weight is fixed along the arcuate portion around the outer periphery edge of the valve disk, the maximum moment can be obtained for the rotation axis.
A flange is formed around the outer periphery of the valve wall, and the weight is disposed along the inside of the flange, and welded to the flange. Thereby, the weight can be positioned by abutting it against the flange.
The flange is further provided with a projection piece, and the weight is welded at the tip of the projection piece. Since the welded part is separated away from the contact surface of the valve wall, the welding heat is not passed to the contact surface.
A flange with a holding piece is formed around the outer periphery of the valve wall, and a convex portion projecting to the back side is formed inside a contact surface of the valve wall with a valve seat, whereby the weight is disposed between the flange and the convex portion along the inside of the flange, and pressed and fixed to the valve wall by caulking of the holding piece. Since the weight is fixed by caulking of the holding piece, the effect of welding heat does not arise at all.
The weight is formed by cutting a strip steel plate. The weight can be produced with excellent yield by cutting the strip steep plate along the arcuate portion around the outer peripheral edge of the valve disk.
The weight is formed by bending a steel material of a predetermined cross section. The weight can be produced with excellent yield only by cutting and bending the steel material.